1. Field of the Invention
The invention relates to valve assemblies, and particularly to an improved assembly for live loading of valve packings.
2. Discussion of Background
Typically, a valve will include a movable stem which is used for moving the valve between two or more positions (e.g., between open and closed positions, or among various positions in the case of a variable valve). Generally, the valve is either a linear valve, in which the valve stem moves in the axial direction of the stem, or a rotary valve in which the valve stem rotates about its axis for changing the valve position. In either case, a packing is often provided to prevent leakage (along the valve stem) of the fluid passing through the valve. In order to maintain a sufficient seal, the packing is typically loaded with a spring force to provide a "live loading". The spring force applies pressure to the valve packing to maintain an adequate seal, and also to compensate for any consolidation or wear of the packing over a period of time.
FIGS. 1 and 2 show conventional arrangements for providing a valve packing with a live loading. As shown in FIG. 1, the loading can be accomplished by a plurality of springs 30 in the form of a stack of belleville springs mounted over the valve shaft or valve stem 32. A compression plate 34 is provided with a recessed portion 34a for accommodating the spring stack, with the compression plate also receiving the valve shaft 32 and threaded studs 36. When the nuts 38 are tightened upon the studs 36, the spring stack is compressed, and imparts a compressive force to the valve packing via ring 31.
FIG. 2 shows an alternate conventional arrangement in which a stack of belleville springs 40 is provided about each of the stud shafts 46. Upon tightening of the nuts 48, the springs 42 are compressed, thereby loading the valve packing.
As the valve is used over a period of time, the packings can wear or consolidate. The consolidation of the packing causes a reduction in the amount of force provided by the belleville springs (i.e., unless the assembly is further tightened), thus risking the possibility of leakage. Accordingly, it is extremely important to provide a load upon the packing which will prevent leakage even after the packing has consolidated over a period of time, while avoiding excessive loading which can result in excessive friction between the packing and the valve stem.
Often, a large loading is required to provide a satisfactory seal with the packings. With conventional arrangements, a high loading undesirably requires an increase in the number of springs. In addition, with conventional live loading arrangements, it can be difficult to determine the point at which a desired load is achieved such that satisfactory initial loading is provided, and such that the loading continues to be sufficient even after the packing consolidates. Moreover, an arrangement as shown in FIG. 1 requires the compression plate 34 to be machined to include a recessed portion 34a in order to reduce the height of the assembly, and/or for proper positioning of the compression plate with respect to the springs 30. The FIG. 2 arrangement is further disadvantageous in that a large number of springs are required, and it is difficult to achieve a desired uniform loading with the separate spring arrangements provided for each stud. The arrangements of FIGS. 1 and 2 are also disadvantageous in that a large number of springs must be added to achieve high loads.
Accordingly, an improved arrangement for providing a live loading of the packing of a valve is needed. Preferably, such an arrangement or assembly should be of a relatively simple construction, and should also be relatively easy to use such that a particular desired loading can be readily achieved in a highly reliable manner, even where the loading requirements are high. Such an arrangement should be suitable for retrofitting existing valves, and should also be capable of accommodating wear or consolidation of the packing, such that a satisfactory load is maintained on the valve packing even after extended periods of use.